The Eagle landed on July 20, 1969. For those who watched the Apollo 11 astronauts park their lunar lander on Tranquility Base—in my case, on a grainy black-and-white television in a small house in the hills above Los Angeles—the fact that Neil Armstrong's "one giant leap for mankind" took place 40 years ago can only come as a shock. Slowly down the ladder went the first human being to step onto the moon, clumsy in his spacesuit, and we knew we were witnessing a moment we would never forget.

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The lunar module that transported Armstrong and Buzz Aldrin to the gritty surface of the moon was a two-section invention built by the Grumman Corporation. The bottom unit consisted essentially of four landing struts equipped with a retrorocket to cushion the descent. It would remain on the moon after the upper section, also rocket-propelled, carried the astronauts back to the command module, piloted by Michael Collins.

In five subsequent lunar landings, the same type of craft would be used to deliver ten astronauts from command modules to the moon. All the landers were left behind, the bases remaining where they touched down. After astronauts returned to the modules, they jettisoned the transport capsules, which crashed into the moon or vanished into space.

Today, lunar lander LM-2 ("LM" is shorthand for Lunar Excursion Module) remains earthbound—a 50-year-old vehicle that never got off the launchpad. It is on display at the Smithsonian's National Air and Space Museum (NASM) in Washington, D.C.

Transporting the Eagle astronauts safely to the moon was the result of a remarkable series of test flights that had begun in 1968 with the earth-orbiting Apollo 7. Early on, NASA planners made the decision to land on the moon from an orbiting craft rather than going directly from earth to the lunar surface. Grumman's solution—a lander with ingenious dual-section construction for separate descent and ascent functions—would prove to be one of the most reliable elements in the Apollo program.

The first lander, LM-1, went into earth orbit on a Saturn rocket on January 22, 1968, for unmanned testing of its propulsion systems. (LM-1 was not intended to return to earth.) The LM-2 was designed for a second unmanned test, but because the first went off without a hitch, another was deemed unnecessary. According to NASM scientist Robert Craddock, the two landers were not designed to be "man-ready"; they lacked safety equipment and other fittings necessary to accommodate astronauts.

During an emergency, a lander, equipped with additional sources of power and propulsion, could also serve as a kind of space lifeboat for astronauts. This is just what happened on the ill-fated Apollo 13 mission in April 1970. As its crew hurtled toward the moon, an oxygen tank exploded, causing damage that knocked out much of the crew's air, electrical and water supplies. The astronauts climbed through a hatch from the command module into the contiguous lander. The lander's rockets supplied the boost needed to guide the space capsule accurately around the moon and back toward earth.

Because landers were designed to be used only in space—not to withstand re-entry into earth's atmosphere—engineers had no need to factor atmospheric friction into their design. Thus, says NASM curator Allan Needell, "the lunar module looks flimsy and gangly—it's a very pure design built for a very specific mission." Every astronaut who went to the moon, he adds, has visited the LM-2 at NASM. "It's obviously the best place for television interviews," he says. "They all think that the lunar module was one of the really unique engineering achievements of the Apollo program."

The LM-2 was built for Earth-orbit tests, not engineered to touch down on the moon; it had to be retrofitted with landing gear in order to replicate the appearance of the Armstrong-Aldrin craft. It has also been recently restored. Its deteriorated gold-tone Mylar sheathing on the descent section and landing struts has been replaced, with extra layers added to recreate the appearance of Apollo 11.

Today, visitors to the Apollo exhibition witness an artifact that looks—with a little help from artful curators—much as Eagle looked when it made that giant leap 50 years ago. When Buzz Aldrin radioed back to us riveted earthlings that "this stands as a symbol of the insatiable curiosity of all mankind to explore the unknown," he was talking about the overall mission. But he might as easily have been referring to the ungainly marvel that made it possible.

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About Owen Edwards

Owen Edwards is a freelance writer who previously wrote the "Object at Hand" column in Smithsonian magazine.